Fuel and Physical Properties of Biodiesel Components

Gerhard Knothe

5.1 Introduction

Biodiesel is an alternative diesel fuel (DF) derived from vegetable oils or animal fats [1, 2]. Transesterification of an oil or fat with a monohy- dric alcohol, in most cases methanol, yields the corresponding mono­alkyl esters, which are defined as biodiesel. The successful introduction and commercialization of biodiesel in many countries around the world has been accompanied by the development of standards to ensure high product quality and user confidence. Some biodiesel standards are ASTM D6751 (ASTM stands for American Society for Testing and Materials) and the European standard EN 14214, which was developed from pre­viously existing standards in individual European countries.

The suitability of any material as fuel, including biodiesel, is influenced by the nature of its major as well as minor components arising from pro­duction or other sources. The nature of these components ultimately deter­mines the fuel and physical properties. Some of the properties included in standards can be traced to the structure of the fatty esters in the biodiesel. Since biodiesel consists of fatty acid esters, not only the structure of the fatty acids but also that of the ester moiety can influence the fuel proper­ties of biodiesel. The transesterification reaction of an oil or fat leads to a biodiesel fuel corresponding in its fatty acid profile with that of the parent oil or fat. Therefore, biodiesel is largely a mixture of fatty esters with each ester component contributing to the properties of the fuel.

Properties of biodiesel that are determined by the structure of its component fatty esters and the nature of its minor components include

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ignition quality, cold flow, oxidative stability, viscosity, and lubricity This chapter discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included in biodiesel standards, although all of them are essential to proper func­tioning of the fuel in a diesel engine.

Generally, as the least expensive alcohol, methanol has been used to produce biodiesel. Biodiesel, in most cases, can therefore be termed the fatty acid methyl esters (FAME) of a vegetable oil or animal fat. However, as mentioned above, both the fatty acid chain and the alcohol functionality contribute to the overall properties of a fatty ester. It is worthwhile to consider the properties imparted by other alcohols yield­ing fatty acid alkyl esters (FAAE) that could be used for producing biodiesel. Therefore, both structural moieties will be discussed in this chapter. Table 5.1 lists fuel properties of neat alkyl esters of fatty acids. Besides the fuel properties discussed here, the heat of combustion (HG) of some fatty compounds [3] is included in Table 5. 1, for the sake of under­scoring the suitability of fatty esters as fuel with regard to this property.

TABLE 5.1 Properties of Fatty Acids and Esters3

Trivial (systematic)

b

name; acronym

Mb’

(°С)

lib’"’

(°С)

Cetane no.

HG7,

Viscosity’ (keal/mol)

Caprylic

16.5

2.39.3

(Octanoic); 8:0

Methyl ester

19.3

.3.3.6 (98.6)""

0.997; 1.19* 131.3

Ethyl ester

-43.1

208.5

1.37 (25°У 1465

Capric

31.5

270

47.6 (98.0)""

145.3.07 (25°)

(Decanoic); 10:0

Methyl ester

224

47.2 (98.1)""

1.407; 1.72* 1625

Ethyl ester

-20

24.3-5

51.2 (99.4)“"

1.99 (25°)J 1780

Laurie

44

1.311

176.3.25 (25°)

(Dodecanoic); 12:0

Methyl ester

5

266766

61.4 (99.1)""

1.95J; 2.4.3* 1940

Ethyl ester

— 1.8fr

16.320

2.88J 2098

Myristic

58

250.5100

207.3.91 (25°)

(Tetradecanoic); 14:0

Methyl ester

18.5

295701

66.2 (96.5)“"

2.69J 2254

Ethyl ester

12.3

295

66.9 (99.3)-"

2406

Palmitic

6.3

.350

2.384.76 (25°)

(Hexadecanoic); 16:0

Methyl ester

.30.5

415-8747

74.5 (9.3.6)-";

3.607; 4.38* 2550

85.9!

Ethyl ester

19.3/24

19110

9.3. Ґ

2717

Propyl ester

20.4

19012

85.0!

Isopropyl ester

1.3-4

1602

82.6!

Butyl ester

16.9

91.9′

2-Butyl ester

84.8!

Isobutyl ester

22.5, 28.9

199°

8.3.6′

Stearic

71

ЗбО’7

61.7,‘

2696.12 (25°)

(Octadecanoic); 18:0

TABLE 5.1 Properties of Fatty Acids and Esters* (Continued)

Trivial (systematic) MP° name; acronym6 (°С)

IIP

(°С)

Cetane no.

HG7,

Viscosity’ (kcal/mol)

Methyl ester 39

442-3747

86.9 (92.1)"";

4.74і 2859

10Ґ

Ethyl ester 31-33.4

19910

76.8,‘; 97.T

3012

Propyl ester

69.9A; 90.9!

Isopropyl ester

96.5!

Butyl ester 27.5

343

80.Iа; 92.5!

2-Butyl ester

97.5!

Isobutyl ester

99.3!

Palmitoleic (9(Z)-

Hexadecanoic); 16:1

Methyl ester

51.0′

2521

Oleic (9(Z)- 16

286100

46. Iа

2657.4 (25°)

Octadecanoic); 18:1

Methyl ester —20

218.520

55a; 59.3!

3.73J; 4.5Iа 2828

Ethyl ester

216-7101

53.9A; 67.8!

5.50 (25°)J

Propyl ester

55.7A; 58.8!

Isopropyl ester

86.6!

Butyl ester

59.8A; 61.6!

2-Butyl ester

71.9!

Isobutyl ester

59.6!

Linoleic (9Z,12Z — —5

229-3016

31.4A

Octadecadienoic); 18:2

Methyl ester —35

21520

42.2A; 38.2!

3.057; 3.65a 2794

Ethyl ester

270-5180

37.1л; 39.6!

Propyl ester

40.6A; 44.0!

Butyl ester

41.6а; 53.5!

Linolenic (9Z,12Z,15Z-

-11

230-217

20.4a

Octadecatrienoic); 18:3

Methyl ester

Ethyl ester —57/-52

ЮдО. ОИ

20.6""; 22.7!

2.65J; 3.14a 2750

Propyl ester

1742’0

26.7a

Butyl ester

26.8a

Ricinoleic (12-Hydroxy — 5.5

24510

9Z-octadecenoic);

18:1, 12-OH

Methyl ester

225-710

15.44а

Erucic (13Z — 33-4

26510

Docosenoic); 22:1

Methyl ester

221-2°

5.9 Iі 3454

Ethyl ester

229-30°

a Adapted from Ref. [4].

b The numbers denote the number of carbons and double bonds. For example, in oleic acid, 18:1 stands for 18 carbons and 1 double bond. c Melting point and boiling point data are from Refs. [5] and [6]. d Superscripts denote pressure (mm Hg) at which the boiling point was determined. e Viscosity values determined at 40°C, unless indicated otherwise.

^HG values are from Refs. [3] and [5].

8 Number in parentheses indicates purity (%) of the material used for CN determination as given in Ref. [7].

6 Ref. [8].

‘ Ref. [9].

; Dynamic viscosity (mPa • s = cP), Ref. [10]. k Kinematic viscosity (mm2/s = cSt), Ref. [11].

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